(i) Field of the Invention
The present invention relates to a resin composition suitable for the manufacture of molded articles which can express a high heat resistance, an excellent dimensional stability at a high temperature and an excellent flexural modulus at a high temperature, and molded articles obtained by molding the resin composition.
(ii) Description of the Related Art
Polyimide resins are usually much more excellent in heat resistance, mechanical properties and chemical resistance than conventional general-purpose engineering plastics, and they are considered to be one kind of superengineering plastic.
The polyimide resins can be classified into thermoplastic resins and thermosetting resins, but the former can easily be molded, and for this reason, their demand increases particularly in recent years.
The employment of the superengineering plastics is significant in that they can replace with metals which cannot be originally easily molded and have large specific gravities. Nowadays, from such a viewpoint, higher performances of heat resistance, dimensional accuracy and productivity than conventional levels have been desired, and an improvement which permits the expression of such performances has been expected.
Regarding a polyimide (1) having a repeating unit represented by the chemical formula 1:
The polyimide (1) having a repeating unit represented by the chemical formula 1 is disclosed in, for example, U.S. Pat. No. 4,847,311 (Japanese Patent Application Laid-Open No. 236858/1987) and U.S. Pat. No. 4,847,349 (Japanese Patent Application Laid-Open No. 68817/1987).
This polyimide (1) is crystalline, so that both of its glass transition point (about 245.degree. C.) and melting point (about 388.degree. C.) of the polyimide are high, and its heat resistance is in a top class among thermoplastic resins. Although being crystalline, however, this polyimide (1) has a low crystallization velocity, and in other words, a long period of time is required for the crystallization of the polyimide. Molded articles obtained by a usual molding cycle, for example, an injection molding cycle of about 30 to 60 seconds are amorphous.
Accordingly, the thus molded articles are excellent in characteristics of dimensional accuracy and flexural modulus, so long as they are used at a temperature lower than the glass transition point.
On the other hand, when the thus molded articles are used at a temperature higher than the glass transition point, the flexural modulus noticeably deteriorates, so that the shape of the molded articles cannot be maintained and hence it is difficult to continuously use them.
If it is attempted to continuously use the molded articles comprising this polyimide (1) at the temperature higher than the glass transition point, the amorphous molded articles should be subjected to a heat treatment to crystallize them. However, this crystallization causes a problem such as the noticeable dimensional change of the molded articles on occasion.
Regarding a polyimide (2) having a repeating unit represented by the chemical formula 2:
On the other hand, the polyimide (2) having a repeating unit represented by the chemical formula 2 is disclosed in, for example, Macromolecules, Vol. 29, p. 135 to 142 (1996).
This polyimide (2) is crystalline, and it has a large difference (about 205.degree. C.) between a glass transition point (about 190.degree. C.) and a melting point (about 395.degree. C.). In addition, it has a very high crystallization velocity, and in other words, a period of time required for the crystallization is very short. In this connection, in the case of the polyimide (1), a difference between the glass transition point and the melting point (about 395.degree. C.) is about 143.degree. C.
This polyimide (2) is crystalline, and additionally, it has the very high crystallization velocity as described above. Therefore, in the molded articles obtained by a usual molding cycle, for example, an injection molding cycle of about 30 to 60 seconds, the crystallization has been substantially completed in the step of the molding, and even when the heat treatment is done, the dimensional change in the molded articles is slight.
In general, the resin having the high crystallization velocity possesses the high mobility of a molecular chain, so that the difference between the melting point and the glass transition point is large. That is to say, the glass transition point of the polyimide (2) is about 190.degree. C., which is about 55.degree. C. lower than that of the polyimide (1). Therefore, there occurs a problem that the flexural modulus, at a medial temperature of 150 to 200.degree. C., of the molded articles comprising the polyimide (2) is lower than that of the molded articles comprising the polyimide (1) which is substantially amorphous and has the high glass transition point.